It is just two weeks since David Cameron delivered one of the best political speeches I have ever heard. In Birmingham, he set out the (in many ways astonishing) achievements of his Government, while capably demolishing the electoral pretensions of Labour.

Mr Cameron concluded with the warning that a vote for Nigel Farage is, in effect, a vote for Ed Miliband. He rightly received rave reviews – all the more glowing when compared with Ed Miliband the week before.

The Prime Minister had provided the Conservative Party with a route map to general election victory. Tory morale has rarely been higher than in the immediate aftermath of that speech. Yet already, it is in collapse.

The Ukip triumph in Clacton has led to a meltdown in party discipline of a kind that I have never witnessed before – not even during the darkest days of the Maastricht rebellion against John Major. Conservative MPs are openly collaborating with Ukip, a political party that is set on their downfall and destruction.

This became apparent on Monday, when Douglas Carswell, the victor of Clacton, returned to the Commons. To my amazement and even stupefaction, he was introduced by two MPs from the very party which he had just deserted.

Nor is that all. I have discovered that a number of Tory MPs actually went to the astonishing length of sending messages of congratulations to Mr Carswell. One of them, James Wharton, sent out an exultant tweet on the news that the Tory candidate had been defenestrated: “Main parties got a kick from the public which should not be ignored. Congrats to Douglas Carswell.” Zac Goldsmith was even more effusive: “Congratulations to Douglas Carswell on his win, and for a wonderful victory speech.” Late yesterday afternoon Goldsmith was at it again, with another tweet praising his former colleague.

The Conservative MEP Daniel Hannan went further. On the day of the Clacton vote, he distributed a generous article about Carswell via social media. Yes, Carswell is one of his closest friends – but this action can only be construed as a veiled expression of open support for a Ukip candidate. On Twitter, I asked Mr Hannan for an endorsement of Giles Watling, the Conservative Party candidate. He was either unable or unwilling to reply. This is part of a pattern: in contemptuous defiance of David Cameron, Mr Hannan has openly called for a merger between the Tories and Ukip.

Normally in British politics, defectors are treated with icy hostility. When Winston Churchill joined the Liberals in 1904, he was cut dead by Conservative MPs, socially ostracised and even blackballed by the Hurlingham Club (he wanted to play polo there).

This Ukip insurgency is often compared with the rise of the Social Democratic Party in the early Eighties. But back then, Labour MPs regarded Roy Jenkins and his supporters as traitors. When Jenkins returned to the Commons after the Glasgow Hillhead by-election in 1982, he was shouted down by his former comrades.

Labour understood that the SDP represented a mortal threat. In the long term, it could only succeed through the elimination of the Labour Party itself. In the short term, Labour MPs realised that every vote cast for the SDP was in effect a vote for Margaret Thatcher. Indeed, historians now acknowledge that the rise of the SDP was perhaps the most important factor behind her landslide victory in the 1983 general election. Hence the ferocity of the Labour hatred – echoes of which survive to this day.

The contrast with Douglas Carswell is stark. And the general Tory benevolence is all the more bewildering because Ukip makes no secret of its intention to destroy the Conservatives. In an interview six months ago, Nigel Farage revealed with disarming candour that he hoped to wipe out the Tories, just as the New Democratic Party in Canada wiped out the Canadian Liberals in 2011.

This is not all. Some Tory MPs are making quiet deals with Nigel Farage to run as joint Tory/Ukip candidates at the election. And in private, Ukip strategists say that they actually want Ed Miliband to become prime minister. They calculate that he will then make such a mess that he will be kicked out by a despairing electorate. Meanwhile, Ukip plans to capture the Conservative Party from within, then use it as a vehicle to take Britain out of Europe.

All of this is familiar territory to anyone who has studied the bitter Labour Party civil wars of 30 years ago. So how do we explain the fact that so many Conservative MPs are, either secretly or openly, on the side of Ukip?

It is helpful to return to a term first used during the Spanish Civil War. Franco’s generals used to claim that they could rely on “fifth columnists” behind enemy lines – secret sympathisers ready to give them overt or sometimes covert support. Some of these were especially dangerous, because they were ready to further Communist objectives without betraying their true allegiance.

Nigel Farage has lined up his own group of fifth columnists inside the Conservative Party – a group who, for various reasons, can be relied upon, whether wittingly or unwittingly, to carry out acts of sabotage, while stopping short of openly allying themselves with the enemy.

When it comes to James Wharton, the Tory MP who publicly danced on the grave of Giles Watling, the humiliated Tory candidate in Clacton, I believe he is a decent man. However, he has a very narrow majority in his Stockton South constituency, and is vulnerable to the Ukip threat. He is only 30 years old, and would surely have been better advised to have entered politics after gaining experience of the real world. His decision to support the Ukip candidate in Clacton can be put down to an artless attempt to appease a political opponent.

His fellow backbencher Jacob Rees-Mogg has defied David Cameron with a call for a Conservative Party alliance with Ukip. Mr Rees-Mogg has powerful views on Europe, and clearly believes that those would be best furthered in this way. There is no doubting his integrity – but Mr Rees-Mogg is even more naive than Mr Wharton.

We now come to Zac Goldsmith – another nice man but politically a shambles. The great cause of his career is the environment, yet Ukip is ostentatiously opposed to any measure designed to head off global warming.

Daniel Hannan is by far the most interesting and significant of Farage’s fifth columnists. He is a highly intelligent man who combines political brilliance with zero judgment, an all-too-familiar combination in high politics. After he failed to answer my question about Clacton, I asked him whether he would support the Conservative candidate in Rochester. I am still awaiting a reply.

Wharton, Rees-Mogg, Goldsmith and Hannan are far from the only Tories who are giving sustenance to Ukip. Their motives often reflect their personal circumstances, and are not always dishonourable. Collectively, however, there is no doubt that the Prime Minister is right: they are helping to bring about a victory for Ed Miliband at the general election.